A radiation detection apparatus that detects radiation radiated from substance is known. For example, Patent Literature 1 (JP 2013-33009A) discloses a gamma camera system. This gamma camera system includes a gamma camera, a distance measuring unit, a position calculating unit, a sensitivity correction information estimating unit, a resolution correction information estimating unit and an image production calculating unit. The gamma camera has a gamma ray detector and a collimator. The distance measuring unit can measure a distance to an imaging object of the gamma camera. The position calculating unit calculates position relation based on the distance between the gamma camera and the imaging object of the gamma camera, which is obtained through scan measurement of the distance measuring unit. The sensitivity correction information estimating unit estimates measurement sensitivity when measuring the imaging object with the gamma camera, based on the position relation obtained from the position calculating unit. The resolution correction information estimating unit estimates the resolution when measuring the imaging object with the gamma camera, based on the position relation obtained from the position calculating unit. The image production calculating unit produces a gamma ray distribution image based on the measurement sensitivity estimated by the sensitivity correction information estimating unit, the resolution estimated by the resolution correction information estimating unit and gamma ray count data detected by the gamma camera.
FIG. 1 is a diagram schematically showing this gamma camera system. The gamma camera system 101a is of a pinhole camera type. Therefore, the viewing angle of the detector 110a to a radiation source 150 is prescribed by the aperture angle of pinhole collimator 140, to limit to 40°-60°. Therefore, in order to measure in the direction of full circumference (360°), it is necessary that the gamma camera system 101a is set on a turn-table 145 and the measurement is carried out a plurality of times (six times if the viewing angle is 60°) while turning the turn-table 145, as shown in FIG. 2. Such a system having the turn-table is disclosed in Non-Patent Literature 1 ((http://www.hitachi-ce.co.jp/product/gamma_detector/).
Also, a Compton camera is disclosed in Non-Patent Literature 2 (http://www.jaxa.jp/press/2012/03/20120329_compton_1.pdf). The Compton camera is a camera which can measure an incidence direction of an incident radiation (e.g. a gamma ray) by using the Compton scattering. The incidence direction may be visualized by using an optional display unit. FIG. 3A is a diagram schematically showing the operation principle of this Compton camera. The Compton camera 101b can determine the energy E1+E2 and arrival direction θ of an incident radiation R1 at a same time, based on the measured position X1 and energy E1, and the measured position X2 and energy E2. In this case, the position X1 and the energy E1 indicate the scattering position X1(x1, y1, z1) and the energy E1 given to an electron from the radiation R1 when the radiation R1 is Compton-scattered with the electron in a scattering layer 110b1. Also, the position X2 and the energy E2 are the absorption position X2(x2, y2, z2) and the absorption energy E2 when the Compton-scattered radiation R2 is subjected to the photoelectric absorption in an absorption layer 110b2. Then, a position or kind of a radiation source 150 and so on are estimated based on the information of them. As shown in FIG. 3B, the incident radiation R1 is scattered at the position X1 of the plurality of scattering layers (Si) 110b1 to give the energy E1, and the scattered radiation R2 is absorbed at the position X2 of the plurality of absorption layers (CdTe) 110b2 to give the energy E2. As a result, the energy and the arrival direction θ of the incident radiation are determined at a same time based on (X1, E1) and (X2, E2). Then, the position or kind of the radiation source 150 and so on are estimated based on the information of them.
The Compton camera 101b can detect radiations from the full circumference direction (viewing angle: 360°; strictly, a solid angle 4π steradian [sr]) theoretically. However, the Compton camera 101b has the structure in which the scattering layer 110b1 is arranged in the front of the camera (in the incident direction) and the absorption layer 110b2 is arranged in the rear of it. In this case, comparing the detection efficiency of a hemisphere in the front of the camera (viewing angle: 180°; strictly, a solid angle 2π steradian) with the detection efficiency of the hemisphere in the rear of the camera (viewing angle: 180°; strictly, a solid angle 2π steradian), the detection efficiency of the hemisphere in the rear of the camera becomes small extremely (e.g. about 1/10). Therefore, the viewing angle in case of practical use of the Compton camera 101b is 180°, and practically, only the radiation from the direction of hemisphere (2π [sr]) can be detected. Therefore, to measure the full circumference direction (viewing angle: 360°; 4π [sr]), for example, the measurement of at least twice need in the Compton camera 101b as shown in FIG. 4.